Electrodeposition of chromium upon a continuous metal strip



United States Patent 3,498,892 ELECTRODEPOSITION 0F CHROMIUM UPON A CONTINUOUS METAL STRIP Edgar J. Seyb, Jr., Oak Park, and Richard E. Woehrle and John G. Neitzel, Livonia, Mich., assignors to M & T Chemicals Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed May 4, 1966, Ser. No. 547,421 Int. Cl. C23b /58, 5/46, 5/06 US. Cl. 20428 2 Claims ABSTRACT on THE DISCLOSURE In accordance with certain of its aspects, this invention relates to a process for electroplating a bright, protective chromium plate which comprises maintaining an aqueous chromium plating bath containing chromic acid and catalyst, including fluoride and sulfate; maintaining an anode in said bath; passing through said bath a continuous strip of a thin-gauge metal cathode at a high velocity of at least about 30 meters per minute; maintaining said continuous strip as cathode in said bath; passing an electric current through said anode, said bath, and said cathode thereby electroplating a bright protective chromium plate on said cathode.

This invention relates to a novel process for electroplating bright, protective deposits of chromium plate on strip metal. More particularly, it is directed to the electrodeposition of bright protective chromium plate onto a continuous metal strip moving at a high velocity.

Chromium plating has found wide use as a means for producing a bright, hard, decorative coating on various articles. It has been recognized that chromium plating of metals results in the improvement thereof particularly insofar as corrosion-resistance, decorative appearance, and adhesion of organic-coatings or enamels are concerned.

It has, however, not heretofore been commercially possible to deposit bright chromium deposits on cathodes moving at high velocities. The high current densities re quired to produce rapid plating on the fast moving cathode cannot be satisfactorily maintained with prior art chromium plating baths and processes and the chromium plate obtained may be dull or blotchy. In other processes, it may be possible to obtain only a chromate film and not a chromium plate. These prior art processes for plating of high velocity continuous strip metal with chromium 'plate, have permitted attainment of chromium plate which may be dull, highly porous, non-continuous, and non-adherent to the basis metal. This dull, non-continuous chromium layer, a combination of chromium metal and chromate film, may have poor aesthetic features and poor binding properties for any subsequent coatings, e.g.

paint or other inorganic coatings.

It is a particular feature of prior art attempts to electrodeposit chromium onto continuous strip moving at high velocity that it has been necessary to pre-treat the strip in an acid bath to etch the surface or remove metal oxides, etc. and to thus insure proper adhesion of the plate to the metal. If the various metal oxides were not removed prior to the electroplating of the chromium, they would introduce contaminants onto the plated surface which would afiect the color of the plate.

3,498,892 Patented Mar. 3, 1970 A typical acid medium which may be used in the pretreatment of the metal strip prior to chromium plating in a chromic acid-sulfate bath may be a 10% aqueous solution of sulfuric acid which treatment was required to msure attainment of a bright chromium plate by prior art processes. I As is Well-known to those skilled-in-the-art, the platmg of chromium onto a high velocity cathode has not heretofore been possible because of the need for complex pretreatment steps, e.g. acid treatment, and because of mechanical limitations of the equipment which limit the speed of the moving cathode. If the cathode is conveyed at an excessive velocity in prior art processes, the chromium plate commonly becomes dull, blotchy, non-continuous, and a chromate film may be produced instead of the desired chromium metal. Prior art attempts to remedy these deficiencies have included activation by strong acid. At desired cathode velocities as high as about meters per minute or higher, the problem may become more critical and it may not be possible to successfully achieve chromium plating. However, such speeds are in common use in the handling of sheet metal including can stock, and it is important to be able to chromium plate at these speeds.

As is also well-known to those skilled-in-the-art, prior art processes for chromium plating onto a continuous moving metal strip included the passing of the metal strip through a plurality of chromium electroplating cells. This passing through the plurality of plating cells may cause current interruption in the plating cycle with resulting defects in the plate. During strip line plating, the metal strip may move from cell to cell and current supplied to the metal strip while it is out of the cell. This current interruption was completely objectionable because it produced a dull, blotchy plate.

Various electromechanical devices have been used in attempts to correct this difficulty. For example, highly complicated special anode arrangements were used to prevent dulling of the chromium plate when using a sulfate bath. This complex anode arrangement included an insulating shield (suspended between the anode and the continuously moving cathode) containing small perforations to serve as a resistance in the current path between the anode and the cathode strip. The purpose of this shield was to gradually increase the current flow and to prevent production of a dull blotchy plate.

A further limitation is imposed upon the continuous electroplating of chromium, wherein a plurality of chromium cells is used, in that the temporary exposure of the metal to the air during discontinuous plating results in passivation of the exposed metal. In order to overcome these various defects, various acid baths may be positioned betweenthe successive plating cells during the plating cycle. For example, dilute sulfuric acid may be used between these plating cells.

These chemical and mechanical necessities have seriously inconvenienced commercial operations and the speed of, production lines has been often limited by these factors. Despite the inconvenience and seriousness of this long standing problem, no practical commercial solution to it may be found in the prior art.

A satisfactory process for plating bright, protective chromium upon a continuously moving cathode of high velocity will contain a minimum number of mechanical and chemical limitations. For example, it is highly desirable that the process avoid intermittent pretreatment steps during plating without producing any detrimental effects upon the bright plated chromium. A satisfactory process should also permit use of a chromium plat ng operation in which there is no drag-in of undesirable impurities by the metal strip cathode. Furthermore, a satlsfactory process must produce an attractive, bright, continuous, protective chromium plate regardless of the'rnetal strip cathode velocity, and particularly at high velocities.

It is an object of this invention to provide a novel process for bright chromium plating on a continuous metal strip moving at a high velocity. It is a further ob ect to provide a process which may be carried out with minimum of mechanical and chemical limitations. Other objects'will beapparent to those skilled-in-the-art from the following disclosure.

In accordance with certain of its aspects, the process of this invention for electroplating a bright, protective chromium plate comprises maintaining a chromium plating bath containing chromic acid and catalyst, including fluoride and sulfate; maintaining an anodein-said bath; passing through said bath a continuous strip of a thingauge metal cathode at a high velocity of at least about 30 meters per minute; maintaining said continuous strip as cathode in said bath; passing an electric current through said anode, said bath, and said cathode thereby electroplating a bright protective chromium plate on said cathode.

The metal strip employed in the practice of this invention may be, e.g. iron, including its alloys such as stainless steel, etc. In practice of the invention, the metal may be a strip or coil of extreme length, typically 30,000 meters or more. The metal may commonly have a thickness of 0.10-2.5 mm., say 0.2 mm. and a width of 0.5-2.0 meters, say 1.0 m. This metal strip may also be preplated with e.g. copper, nickel, etc.

It is a particular feature of the novel process of-this invention that it may be possible to chromium plate metal without any pretreating or conditioning. For example, if a coil or steel strip is received free of superficial contaminants (such as oil or grease or gross rust areas) it may be subjected to electroplating in the as-received condition.

If, however, the continuous metal strip bears a film of grease or oil, it may be desirable to anodically (or cathodically) pre-treat in an alkaline electro-cleaner. This alkaline electro-cleaner may contain 30-225 g./l. of active ingredient. The alkaline electro-cleaner bath may contain inorganic salts, e.g. hydroxides, silicates, borates, phosphates, etc., typically sodium hydroxide. Surfactants, including sodium alkyl sulfates, may be present. The bath may be operated at 60 C.100 C., say 80 C. with anodic current density of 3-10 a.s.d., say 5 a.s.d, There may thus be removed oils and greases.

When the strip bears a substantial portion of gross rust areas, it may be pre-treated by acid-immersion. The acid used may comprise 1% to 20%, say of a suitable mineral acid, typically hydrochloric acid or sulfuric acid.

The chromium plating bath which may be employed in practice of this invention may be an aqueous solution containing 10 g./l.-50O g./l., typically 150 g./l.-300 g./l., say 225v g./1. chromic acid CrO The bath may also contain catalysts, as herein defined, the ratio of chromic acid to catalyst typically being from 10:1 to 150:1, preferably 50:1 to 100:1, say 80:1. The ratio as the termis used in this application, refers to the ratio of C10 (solo wherein each of the quantities is specified in g./l. The symbol F- may refer to the fluoride-containing catalyst which may be, e.g. fluoride se or silicofluoride SiF or other fluoride-containing ions as set forth infra. v is an integer which may be two for fluoride complexes, e.g.

SiF and 0.25 for fluoride ion se. Typically the concentration of catalyst in the bath may be 0.7-50.0 g./l., preferably 1.5-6.0 g./l. The catalyst may include sulfate Sof, typically provided as sulfuric'acid, strontium sulfate, etc. The sulfate ion may typically comprise 5%85%, preferably 10%-50%, say 30% of the total catalyst concentration. It is a particular feature of this invention that the chromium-plating bath contain fluoride, typicallysupplied as fluoride ion F- or as a complexfluoride. Typically complex fluorides may include fluoroaluminates, fluorozirconates, fluorotitanates, fluoroborates, etc. The preferred fluoride ion may be' silicofluoride SiF preferably provided as potassium silicofluoride. The preferred catalyst may comprise fluoride in amount of 15%95%, typically 50%90%, say 70% of the total catalyst;

In-the, practice of this invention, the temperature of plating may be 27 C.-90 C., typically 50 C'.70 Ci, say C.

t A typical illustrative bath which may be used in practice of this invention may include the'following wherein, unless otherwise indicated, all parts are parts by weight;

TABLE I Component Minimum Maximum Preferred C r0 150 300 i 275 S0 O. 75 3.0 1. 4 SiFr 1. 5 6. 0 2. 8

* Typically added as sodium sulfate.

** Typically added as sodium silicofiuorlde.

The typical illustrative bath of Table I used in the practice of this invention may be formed by dissolving the noted solid compositions in aqueous medium to form a bath containing, e.g. 150 g./l. to 300 g./l. of CrO and corresponding quantities of the other components. H

In practice of the process of this invention, a continuous strip of thin gauge metal, typically so-called can stock, a steel strip having a thickness of 0.1 to 0.3 mm. and a width of 0.5-2.0 meters may be passed through a series of plating baths. Commonly the strip may be passed through 3-25, typically 5 baths. As the strip is passed between the baths, it may pass over rollers on routev to the next bath. During its passage from one bath to the next, the portions of the strip may thus be out of contact with the path and may have little or no electrical charge thereon. During its stay in each of the baths, the strip may :be maintained at cathodic potential by ,voltage impressed thereon. There may also be maintained in each tank appropriate anodes from which electric current may be passed from the anode to the continuously moving strip cathode. v I

The velocity at which the continuously moving high velocity metal strip cathode may be moving through the chromium plating baths may typically be 30-1800 meters per minute. It may be possible to satisfactorily eflect chromium plating at velocity lower than this, but the advantages inherent in a strip line plating operation under normal commercial conditions are such that the line would normally operate at velocities at least as high ,as these. Commonly the velocity of the cathode may be 30- 1800 meters per minute. Satisfactory chromium plate may also be effected at higher velocity, but commercial conditions for strip line plating may not normally be carried out at velocity appreciably higher than 1800'meters per minute. It is a particularly unexpected feature of this invention, that a desired chromium plate may be satisfactorily attained when operating at speeds of 150-1800 meters per minute, typically 500 meters per minute (m./m.). It has heretofore not been commercially possible to attain the desired chromium plate at velocity of, e.g. -650 m./m., preferably 300-610, and it is an outstandingly unexpected feature of the process of this invention that the chromium plate produced at this velocity is satisfactory in all respects.

During plating in accordance with the process of this invention, the current density which may be employed may be 32-1100 amperes per square decimeter .(a.s.d.),

In accordance with the preferred embodiment of this invention, the high velocity strip may be maintainedwithin the chromium plating baths for 0.05-10 sec., typically I 0.25- sec., preferably 0.75 sec.

As is apparent to those skilled-in-the-art, the thickness f of the chromium plate produced may be varied. Typically it may be desired to produce a chromium plate having a thickness of 0.0050.15 micron, say 0.01 micron. Typically it may be found that the thickness of 0.001-025 micron may be obtained on the moving cathode.

The product produced by the process of this invention may be a strip of metal bearing a bright protective chromium plate which may be found to be decorative and corrosion resistant. However it may be desired to further treat this bright decorative chromium plate to permit attainment of still greater degrees of added protection and to perm-it outstanding adhesion to lacquer which may be subsequently applied thereon. Further treatment may include immersing the chromium plating strip in an aqueous solution containing a dichromate preferably an alkali metal dichromate, typically sodium dichromate present inamount of 7-30 g./l., say 18 g./l. together with 0-20 g./ 1., say 18 g./l. of a phosphate, typcally monosodiumphosphate. During treatment, the chromium plating strip may obviously be subjected-to a cathode current densityof 0-45 a.s.d., say 1.8 a.s.d. for 0.1-1.0 sec. at temperature of 50 C.90 C., say 70 C. v r l It is a particular feature of the process of this invention that the chromium plated strip thereby produced (whether or not subjected to post-treating) maybe characterized by its outstanding properties. For example, if the-chromium plated steel strip be thereby coated with a general purpose (polybutadiene base) pigment enamel, commonly used to coat metal cans in which meat and fish may be canned, it may be found that outstanding properties may be observed. If such a sample be subjected to the Pork Pack test, it may be found that an unexpectedly high degree of adhesion of enamel to the plated metal may be observed. In this test, the enamel sample may be immersed in a can of cooked pork. The can may be sealed while hot and maintained for three hours at pressure of 215 gramsper square centimeter absolute steam pressure and then watercooled to room temperature.'.-After 48 hours, the canmay be opened and the samples examined for staining. Adhe-. sion of the lacquer to the chromium plate may be tested by scribing an X throughthe'enamel and then applying No. 610 Scotch Brand tape .firmlyto-the surface. As the tape is removed with a suddenpull, the enamel may be removed along the scribing'line.and'thus indicatesthe adhesion'of the enamel to the chromium plate. l. i i The chromium plate produced by the novel process of this invention may normally befound. to .be outstanding with respect to adhesion as determined by this fPork Pack test. I

A second test'which may be used to demonstrate the outstanding properties of this. novel. chromium plated steel strip may be the Beer Can Drop test. In this test, a standard beer can base coat (oleoresin varnish) enamel may be applied over platedmetal'and baked at 210i C. for 10 minutes; then a beer can top coat (vinyl base) enamel may be applied and baked a t 171"."C, for '10 minutes. The sample may then be-placed in boiling water for 15 minutes and then tested withtScotch Brand tape for adhesion. It may be found that containers fabricated from the. chromium plated strip of this invention may satisfactorily pass this test, i.e. the enamelwill adhere substantially completely to the chromium plated surface.

A further test of the chromium metalcoating usinga metal strip in accordance with the process "of this invention may be the Humidity Test AfterSevere Deformation, In this test, the chromium platedspecimenmay be bent over angle of and flattened back on itself. The sample may then be subjected to high temperature and high humidity (dry bulb 71 C.-wet bulb 63 C.) and examined every 24 hours. Twenty-four hours exposure .to thistest may equal approximately 1.5 years of .warehouse-exposure. Commonly the chromium plated steel prepared in accordance with the process of this invention may be totally free of rust for periods of time greater than about 48 hours and frequently as long as 72 hours, this being equivalent to 4.5 years of warehouse exposure.

The novel process of this invention will be more apparent .to those skilled-in-the-art from inspection of the following examples showing preferred embodiments of this invention (wherein, unless otherwise indicated, all parts are parts by weight).

EXAMPLE 1 In this example a strip of commercial can stock of 0.25 mm. thickness was processed through a chromium plating bath at a high velocity of 305 meters per minute. The plating bath contained 247 g./l. chromic acid CrO 1.1 g./l. sulfate 50.; and 2.1 g./l. of silicofluoride SiF Bath temperature may be 65 C. and current density 157 a.s.d. A steel anode was maintained in the bath and the current permitted to pass for 2.2 seconds to produce an electroplate of chromium metal having a thickness of 0.06 micron.

The so-formed chromium plate may be post-treated by dipping into an aqueous solution containing 18 g./l. of sodium dichromate and 18 g./l. of monosodium phosphate. The so-treated strip may then be dip-coated with a general purpose can enamel containing zinc oxide (C-46 enamel) and baked.

The specimen may then be subjected to the Pork Pack test described supra and found to be totally satisfactory in that it may show no visible sulfide staining nor apparent change'iner'iamel appearance and no enamel peeling at the scribe line on the sample. When subjected to the Beer Can Drop test, the sample was also outstanding in that it showed total adhesion of enamel coating to the chromium plate. When subjected to the Humidity Test After Severe Deformation, the sample showed perfect surface after 48 hours and only a small rust spot on the tight bend of the metal-plated strip after 72 hours.

EXAMPLES 2-26 In this example a strip of metal can stock formed of low carbon steel of 0.25 mm. thickness may be precleaned in commercial alkaline electro-cleaner for one minute at anode current density of 5 a.s.d. The strip may then be maintained in a standard rotating cathode cell including a rotating cathode support adapted to be maintained within a chromium plating bath. The cathode support may typically be 10 cm. in diameter and 15 cm. long; and it may be maintained in the plating bath with its longer axis in vertical position. The cathode support bears means for rotating the cathode thereon and means for rotating the cathode suppoit around its' 'v'ertical axis at predetermined velocity. Surrounding the cathode support of the test sample may-be a steel anode rack of generally cylindrical shape having an inside diameter of 15 cm. and a-height of 15 cm. In practice, the metal cathode strip, shaped in the form of'a cylinder, may be fastened to the cathode support by means of a cathode screw holder. This rotating cathode may be immersed in a chromium plating bath wherein chromium metal may be deposited on the rotating cathode. 7

Conditions of plating, including concentration in the chromium bath (in grams per liter) of chromic acid, of sulfate supplied as sodium sulfate, of fluoride (or fluoride complex, if present), temperature, current density, velocity of the strip, and plating time may be as set forth in Table II. After completion of chromium plating, the thickness of the chromium plate was determined and the appearance of the chromium was observed. These latter data are set forth in Table III.

that with an interrupted current (Example the plate prepared using a bath having no fluoride was an unde- TABLE II Velocity, Plating Fluoride, Temp., C.D., meters Time, Ex. CrOa, gJl. S04, g./l. g./l. C. a.s,d. per min. sec.

239 2.35 None 46 28. 4 93 9. 8 258 1. 95 None 65 54 183 6. 7 222 0. 95 SiFa, 2 1 60 54 183 6. 6 250 2. 5 None 46 29. 7 93. 3 267 1. 34 3. 36 46 29. 7 93. 3 10 250 2. 5 None 65 119 312 3 275 1. 38 AlFs, 2.75 65 108 312 3 275 1. 38 SiFe, 1.37 65 105 312 3 275 98 SiFB, 1.97 65 105 312 3 275 1. SiFa, 4.59 65 112 312 8 75 37 SiFe, 0.75 63 123 152 2 350 1. 75 SiFa, 3.5 65 107 312 3.1 450 2. Silk, 4.5 65 107 312 3. 0 250 l. 25 SiFo, 2.5 88 565 312 0. 45 250 2. 38 SiFo, 1.25 46 157 468 2 250 1. 25 SiFu, 2.5 88 1, 058 312 0.25 250 2. 38 811%, 1.25 46 53 175 6 273 1. 38 AlFs, 4.12 65 156 467 2 250 None AlFo, 4.0 65 62 467 6 275 1. 38 ZrFe, 8.4 65 111 312 3 275 1. 38 TiFs, 5.5 65 110 312 3 275 1. 38 B1, 2.75 65 107 312 3 275 1. 38 F, 1.38 65 110 312 3 250 1. 25 SiFs, 2.50 88 565 312 0.08 280 1. 4 SiFo, 5.2 65 124 965 1. 98

In the example set forth in Table II, Examples 2-4 were not subjected to pre-treatment by dipping in 10% sulfonic acid for 5 seconds; Examples 5-26 were sotreated. Examples 5-9, 11, 13-14, 1920, and 23 were subjected to an interrupted current, i.e. the current was maintained for 0.5 second and thereafter the current was turned off for 0.2 second, and thereafter the above cycle was repeated to give the total plating time. The results It will be apparent from inspection of Tables II- and III that the novel process of this invention permits attainment of the following outstanding results.

It will be observed, for example, that control Examples 2 and 3 (which were not conducted in accordance with the practice of this invention) wherein the chromium plating bath contained no fluoride ion, did not permit attainment of desired bright chromium plate. Specifically this control example yielded a strip which was covered with a dark chromate film which did not have the brightness associated with a bright chromium plate.

Example 4, showing chromium plating under the conditions of this invention from a chromium plating bath containing 2.1 g./l. of silicofluoride ion (provided from potassium silicofluoride), may permit attainment of a bright, protective, chromium plate.

From inspection of Examples 5 and 6 it may be noted sirable, dull matte plate. From Example 6, it will be apparent that the novel process of this invention permits at tainment of a bright, protective, chromium plate. Similarly, Example 7 shows that with an interrupted current in the bath without fluoride, one obtains a chromate deposit rather than a bright chromium deposit. Examples 8 to 26 disclose illustrative processes falling within the scope of the present invention wherein in each case it may be possible to obtain a. bright, protective chromium plate in periods of time as little as 0.09 second using various fluorides including fluoaluminate AlF (provided as potassium alumino fluoride); fluosilicate SiF (provided as potassium fluosilicate); zirconium fluoride ZrF (provided as potassium fluozirconate); fluotitauate TiF (provided as potassium fluotitauate); BR, (provided as potassium fluoborate); and fluoride P, so (provided as potassium fluoride).

As is apparent from the above, it may be possible to prepare a bright, protective deposit of chromium plate on a metal strip moving at high velocity through a chromium plating bath which may, for example, be under conditions such that the current is interrupted as the strip goes from one bath to the next. It will be further apparent that the novel chromium plate so obtained may be characterized by desirable thickness in minimum time; and particularly that these deposits may be obtained at velocities as high as 60 meters per minute and as high as 1800 meters per minute. It is further apparent that the deposits so obtained may be highly satisfactory when measured by the standard tests to which so-called can stock may be subjected.

We claim:

1. The process for electroplating a bright protective chromium plate which comprises maintaining an aqueous chromium plating bath containing 10 g./l.500 g./l. chromic acid and catalyst including fluoride and sulfate present in an amount to give a ratio of chromic acid to catalyst from 10:1 up to :1 and wherein the fluoride comprises 15-95% of the total catalyst; maintaining an anode in said bath; passing through said bath a continuous strip of a thin-gauge metal cathode at a high velocity of at least about 30 meters per minute; maintaining said continuous strip as cathode in said bath; passing an electric current through said anode, said bath, and said cathode thereby electroplating a bright protective chromium plate in thickness of 0.005-0.15 micron in 0.05-10 seconds on said cathode.

2. The process as claimed in claim 1 for electroplating a bright protective chromium plate wherein the fluoride 9 10 is silicofluoride and wherein the velocity of the thin-gauge OTHER REFERENCES metal cathode through the bath is at least about 120 meters H Jones et all: American Electroplaters, Technical Per mmute' Proceedings, vol. 51, pp. 24-28 (1964).

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